The present invention relates, in general, to piezoelectric sensors and actuators constructed of polyvinylidene fluoride laminates for use in both sensing and controlling complex mechanical movements.
In 1969, H. Kawai, discovered that the organic polymer, polyvinylidene fluoride (PVDF), exhibits a strong piezoelectric effect. Recently, PVDF has been processed into thin films which are flexible and lightweight, and can be utilized to both sense and generate bending or flexing movements. As an example, in U.S. Pat. No. 4,565,940 to Hubbard, PVDF film was applied to a beam, and employed to control or dampen vibrations in the beam. Through the application in the film of a voltage of proper amplitude and phase, strain is induced in the film of appropriate phase, amplitude, and frequency, to dampen vibrations in the beam.
Although the Hubbard system works well for controlling simple bending motions or vibrations, it is not applicable for controlling more complex motions, which include not only bending, but stretching and torsional movements as well. This is because the PVDF film only generates normal stress and strain with respect to its principal axis when voltage is applied across its thickness direction. Consequently, only movement induced by normal stress or strain such as bending can be sensed or generated by the film. The present invention, on the other hand, provides integrated/distributed PVDF sensors and actuators which can be used to sense and control complex mechanical motions, or components of complex mechanical motions.
The possible applications of these sensors and actuators are endless. The construction and operation of large space structures, computer magnetic readers which have to match the rapid density increases in compact storage technology, etc., pose new and challenging active control problems. These structures are mechanically flexible because of their low rigidity and the low natural damping in light weight construction materials. Therefore, active feedback control becomes essential to the construction and operation of these structures. An important question in the control of flexible structures is whether it is possible to control the structure by means of on-board computers performing real-time computations. Therefore, the application of point sensors in the active control of flexible structures poses two problems. First, if only a few point sensors are installed, then there will not be enough information to reveal the system responses. Second, if a large number of sensors are installed, the on-board real time computation requirement becomes serious. A natural question that arises from this understanding is what is the possibility of fabricating some sort of integrated/distributed sensor which transfers the on-board real time computation efforts to the sensor design processes.
Similarly, the application of point actuators has its own problems. First, external actuators are not applicable for most free-fly space structures. Second, the number and the weight of actuators can affect the dynamics of a flexible structure. In other words, the proof-mass actuator is not an elegant way of changing the system dynamics of a flexible structure. The actuators possess the same requirements as the sensing aspects of the control algorithm. An integrated/distributed actuator could be the solution.